Magazine for handling stripping foils in a particle accelerator

ABSTRACT

Thin foils for stripping a particle beam are stored in a magazine that is operable remotely to display an individual foil, release it when it is spent, and repeat this process. The magazine is operable in the high-vacuum, high-radiation environment in the interior of a particle accelerator, and it uses the magnetic field of the accelerator to operate the display and dropping mechanism.

United States Patent G0rka,'Jr. Feb. 18, 1975 [54] MAGAZINE FOR HANDLINGSTRIPPING 2,964,710 12/1960 Stone et a1 328/233 X FOILS [N A PARTICLEACCELERATOR 3,353,107 11/1967 Van De Graaff 328/233 Inventor: Andrew J.Gorka, Jr., Naperville,

Ill.

The United States of America as represented by the United States AtomicEnergy Commission, Washington, DC.

Filed: May 30, 1974 Appl. No.: 474,550

Assignee:

US. Cl 328/233, 313/152, 313/359 Int. Cl. H05h 7/00 Field 0f Search328/228, 233; 313/359,

References Cited UNITED STATES PATENTS 6/1953 McMillan 313/149 X 54EXTERNAL SOURCE r Primary Examiner-Palmer C. Demeo Attorney, Agent, orFirm-John A. Horan; Arthur A. Churm; Donald P. Reynolds [57] ABSTRACTThin foils for stripping a particle beam are stored in a magazine thatis operable remotely to display an individual foil, release it when itis spent, and repeat this process. The magazine is operable in thehigh-vacuum, high-radiation environment in the interior of a particleaccelerator, and it uses the magnetic field of the accelerator tooperate the display and dropping mechanism.

12 Claims, 7 Drawing Figures PAIENIEB FEB 1 81975 SHEEI 10? 3 PATENTEBFEB 81375 SHEET 2 OF 3 EXTERNAL SOURCE MAGAZINE FOR HANDLING STRIPPINGFOILS IN A PARTICLE ACCELERATOR CONTRACTUAL ORIGIN OF THE INVENTION Theinvention described herein was made in the course of, or under, acontract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to the means ofcharge exchange known as stripping. It provides improved opera tion ofvarious kinds of accelerators of charged particles.

Stripping is a well-known technique in the field of charged-particleaccelerators. As a part of the process of acceleration, it is oftendesirable to change the state of charge of the particles in a beam. Thisis done by causing the particles to collide with other particles. Oneway of stripping is to introduce molecules of gas into the path of thebeam. Another way is to place a thin'foil in the path of the beam. Theobjective is to assure that a high percentage of the number of particlesin the beam makes a predetermined number of collisions with strippingmolecules, and that a small percentage collides with a different numberof stripping molecules. The latter objective requires that, if a foil isused, it be very thin.

Various materials are used to strip. In an example of the instantinvention, the stripping atom is carbon which is disposed in the beampath as a very thin foil of an organic polymer. The particular foil usedin the practice of the present invention is a thermally stabilizedpolymer of p-xylene, approximately 5800 A in thickness, formed by vapordeposition. Criteria for its selection included the following. Amaterial for stripping should comprise atoms of low atomic numbers,preferably at and below the atomic number of carbon. Such a materialmust withstand the energy of the beam without melting or decomposing. Itmyst have sufficient tensile strength to support itself in pieces thatspan a cross-section larger than the cross section of the beam to bestripped. It must maintain this tensile strength long enough to providea reasonable period of accelerator operation, of the order of at leastan hour, before requiring replacement. It must be self-supportingwithout a backing and without a framework at all edges. It mustwithstand the vacuum environment in the interior 'of a particleaccelerator and it must not produce or release particles to comprise avirtual leak.

The stripping foil must be disposed in the beam for stripping at apredetermined time and the means of disposing the foil must permitremote operation. There must be a way to remove and replace foil thathas been damaged by radiation, and this way should minimize acceleratordowntime. The means for disposing foils inust be unaffected byradiation, high vacuum, and the magnetic field of the accelerator.

It is an object of the present invention to provide an improved means ofstripping a beam of charged particles.

It is a further object of the present invention to provide a means ofdisposing a thin film of polyparaxylylene in a beam of charged particlesto effect charge exchange.

It is a further object of the present invention to provide a remotelyoperable means of placing and replacing stripping foils in a particlebeam.

It is a further object of the present invention to provide a means ofplacing stripping foils in a particle beam under conditions ofradiation, high vacuum, and high magnetic field.

SUMMARY OF THE INVENTION A remotely operable magazine stores, displays,and releases thin foils of poly-paraxylylene to effect charge exchangein a particle beam. Each foil is connected at each of two opposite sidesto a rod which projects beyond the sides of the foil. The projectingportions of the rods are held in notches in the magazine, which stores aplurality of such foils. Operation comprises indexing the magazine torelease one rod, dangling the foil by the other rod to place the foil inthe beam. A second indexing step releases the second rod to drop thefoil. Successive indexing steps repeat the process to display and removea plurality of foils, one at a time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional top viewof the magazine of the present invention.

FIG. 2 is a sectional side view of the magazine of FIG. 1, taken alongsection lines 2-2.

FIG. 3 is a top view of the coil of FIGS. 1 and 2.

FIG. 4 is a sectional view of the magazine of FIG. 1, taken alongsection lines 44, showing a dangling foil.

FIGS. 5, 6, and 7 are expansions of the sectional side view of FIG. 2showing three successive states of the sequence of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a partialsectional top view of the magazine of the present invention, and FIG. 2is a sectional view of the magazine from the side, taken along sectionline 22. In FIGS. 1 and 2, shelves 10 support a plurality of foilassemblies 12. Each such foil assembly 12 comprises a foil 14 which isattached to a first rod 16 and to a second rod 18. Shelves 10 supportfoil assemblies 12 by their first rods 16 and second rods 18. Shelves 10comprise parts of support frame 20, and notched indexing frame 22 issupported on shelves 10. The notches 24 in indexing frame 22 engage theprojecting ends 26 and 27 of first rods 16 and second rods 18 so assuccessively to (I) hold first rods 16 and second rods 18 on shelves l0;(2) upon once moving, drop a first rod 16 from a shelf 10 to dangle afoil 14 from a second rod 18 into beam path 19; and (3) upon movingagain, to drop the second rod 18 from a shelf 10, thus dropping the foilassembly 12. This cycle is repeated to supply new foils 14, one at atime.

To move indexing frame 22 with respect to support frame 20, ratchetmechanism 28 is actuated by applying a pulse of electric current to coil30. This generates a magnetic field that interacts with the acceleratormagnetic field 32 to produce a magnetic force. This magnetic forcesqueezes coil 30, moving ratchet drive rod 34 in the direction of arrow36. Removal of either or both of the current in coil 30 and theaccelerator magnetic field 32 relaxes the magnetic force on coil 30,which returns under its own elasticity to its original position. Fixedspring 38 is placed under support 40 to maintain contact of ratchetmechanism 28 with ratchet slots 42. Screws 44 affix a bracket 46 thatmaintains the near side 48 of coil 30 in a fixed position with respectto support frame 20. Electric current is supplied to coil 30 throughwires 50, which connect to upper sliding contact 52 and lower slidingcontact 54, which in turn are connected to external source 56.

The position of coil 30 is shown diagrammatically in two views in FIG.3, which is a top view of coil 30. In FIG. 3, near side 48 is in a fixedposition, and far side 66 is free to move under the influence of themagnetic force between current flowing in coil 30 and the acceleratormagnetic field 32, here shown as the conventional view of an arrowdirected into the paper. The solid lines indicate coil 30 in a relaxedposition; the dotted lines show coil 30 squeezed. The motion associatedwith the change from the solid line to the dotted line actuates ratchetmechanism 28.

Further understanding of the invention may be had from FIG. 4, which isa sectional view of the magazine of FIG. 1 taken along section lines 44.In FIG. 4, two shelves are seen to support a foil assembly 12 by ends 27of a second rod 18. First rod 16 has been dropped previously, allowingfoil 14 to dangle from second rod 18 in beam track 21. FIG. 4 is takenthrough the center of a notch 24 in indexing frame 22. FIG. 4 also showsthat support frame 20 is itself held in position by projections 58,which rest on a carrier 60. In operation, the carrier 60 is attached tothe vacuum wall 62 of a particle accelerator. Upper fixed contact 64engages upper sliding contact 52, and lower sliding contact 54 isengaged by carrier 60. Upper fixed contact 64 and carrier 60 areconnected electrically to external source 56 to effect an electricalconnection from outside the vacuum wall 62.

Further understanding of the present invention may be had from FIGS. 5,6, and 7, which are expanded views of portions of the sectional view ofFIG. 2 showing successive steps in the sequence of operation. FIG. 5 isthe standby condition. Four foil assemblies 12 are shown loaded in themagazine, held in place by notches 24 on shelves 10. These notches arefurther labeled a through It to indicate alphabeticallyv the sequentialorder in which foil assemblies 12 will be dangled and dropped. Each foilassembly 12 is supported by a first rod 16 and a second rod 18, whichrest on shelves 10. Indexing frame 22 is moved with respect to supportframe 20 by ratchet mechanism 28. Coil is capable of carrying anelectric current that generates a magnetic field to interact withaccelerator magnetic field 32, actuating ratchet mechanism 28. In FIG.5, coil 30 carries no current, and as a result near side 48 and far side66 are separated by an equilibrium distance A. In FIGS. 6 and 7, coil 30is energized with electric current flowing in a direction that, togetherwith the direction of accelerator magnetic field 32, squeezes near side48 and far side 66 together. In FIG. 6, near side 48 and far side 66 areseparated by a distance B, which is less than A. This represents partialactuation. Full actuation is shown in FIG. 7, in which near side 48 andfar side 66 are separated by a distance C which is less than B.

Referring to FIG. 5, ratchet mechanism 28 can be seen to have threeparts, a driving leaf 68, a stopping dog 70 and a forward leaf 72.Spring tension of all three parts is maintained by support 40 whichtends to hold them into ratchet slots 42 in indexing frame 22. In FIG.5, driving leaf 68 and forward leaf 72 engage two different ratchetslots 42. However, stopping dog 70 is kept from engaging a ratchet slot42 by cam 76, which is attached to and moves with driving leaf 68. Cam76 raises stopping dog 70 to clear ratchet slot 42. This is evident fromFIGS. 6 and 7 in which ratchet mechanism 28 is operating. In FIGS. 5, 6,and 7, driving leaf 68 is connected by arm 78 to far'side 66 of coil 30and moves with far side 66. Stopping dog 70 and forward leaf 72 are bothconnected by arm to support frame 20, and maintain their position withrespect to support frame 20. The beginning of such motion appears inFIG. 6, in which driving leaf 68 has been moved in the direction of thearrow 69, driving indexing frame 22 with it. Stopping dog 70 is beingheld up by a land 82 between two ratchet slots 42. Forward leaf 72 isbent to permit motion of indexing frame 22 in the direction of the arrowbut to catch in a ratchet slot 42 and prevent motion in the directionopposite to the arrow.

The completion of one cycle of motion of indexing frame 22 is shown inFIG. 7, in which driving leaf 68 has reached its maximum extension inthe direction of the arrow 71. Further motion in this direction isprevented by stopping dog 70, which has dropped into a ratchet slot 42.Forward leaf 72 has also dropped into a ratchet slot 42, and will holdindexing frame 22 from moving in the direction opposite to the arrowwhen the current in coil 30 is removed and driving leaf 68 returns tothe position shown in FIG. 5. If it is necessary or desired, the returnof driving leaf 68 and hence indexing frame 22 to the equilibriumseparation distance A can be assured by applying to coil 30 a restoringelectric current in a direction opposite to the direction that actuatedthe ratchet mechanism 28.

The motion of indexing frame 22 relative to support frame 20 movesnotches 24 successively past a first edge 84, then a second edge 86, ofshelves 10, first dangling a foil assembly 12, then dropping it. In FIG.5, four foil assemblies 12 are stored in a ready position. The actuationof coil 30, already described, has produced in FIG. 6 some motion offrame 22 that has half uncovered notch 24a. Notch 24b and all higherlettered notches are still fully covered. Operation of ratchet mechanism28 has resulted in FIG. 7 in completion of the motion of frame 22, andnotch 24a is fully uncovered. This has released a first rod 16 fromnotch 24a,

and the foil assembly 12 dangles from its second rod 18 in notch 24b.This foil assembly 24 will-strip a particle beam passed through it alongbeam path 19 until radiation damage or any other reason causes one towant to replace it. Ratchet mechanism 28 is then actuated again, causingnotch 24b to be uncovered. The foil as sembly 12 that was hanging fromnotch 24b drops from the magazine. Another actuation of ratchetmechanism 28 uncovers notch 24c, dangling a fresh foil. This cycle isrepeated until all the foils are used, at which time a fresh magazine isintroduced.

The present invention has been built to strip negative hydrogen ions toimprove injection into the Zero Gradient Synchroton at Argonne NationalLaboratory. Foils were prepared by vapor deposition of poly-pxylylene toa thickness of the order of 5800 A. These were trimmed to rectangularsheets approximately 4 /2 X 2 /zinches. The short ends were attached tononferrous rods, specifically 3 inch pieces of 16 gauge copper wire, tomake the foil assemblies of the present invention. The copper wire wasattached to the foil at the short sides by adhesion to clean surfaces,using heat. The magazine of the present invention was designed to storeand display 12 foils at a time. Both the support frame and the indexingframe were made of an epoxyfilled glass fiber material selected for itsradiation resistance, its ability to withstand vacuum, and itsnonmagnetic properties. This material also does not tend to cohere invacuum, so that the two frames slide with respect to one another in thevacuum environment of the interior of a particle accelerator.

The ratchet mechanism comprised driving pieces made of thin strips ofnonmagnetic stainless steel attached to an actuating coil of about 100turns of No. 32 wire insulated with Formvar, a trade name for a vinylacetal resin varnish insulator. The magnetic field set up by a currentflowing in this coil interacted with the magnetic field used forsynchrotron acceleration in the Zero Gradient Synchrotron to operate theratchet mechanism. When this apparatus was used, the spring action ofthe coil was sufficient to reset the ratchet. In operation it might bedesirable to provide a positive return of the ratchet. This could easilybe done by applying a pulse of current in the opposite direction to theactuating coil.

The apparatus disclosed herein met the requirements for stripping,earlier mentioned as including the use of a very thin foil to providesingle atomic collisions with light atoms in limited numbers. The desirefor single collisions necessitated a thin foil which contributed to thehandling problems solved by the present invention. Foils as disclosedherein have operating lifetimes of the order of an hour or more, and thetime necessary to change foils using the present invention is less thanseconds. This provides excellent stripping operation with acceptableamounts of accelerator downtime. Each magazine is capable of providingstripping foils over a period of the order of at least 12 hours. This isadequate for many types of operation. A transport system designed inassociation with the apparatus of the present invention but not claimedtherewith provides remotely controllable supplies of a plurality of suchmagazines which extends the possibility of operation to an indefiniteperiod of time. The transport system supplies a magazine from outsidethe accelerator, positions it above the beam in external electricalcontact with the current supply for the ratchet mechanism, and thusprovides for the operation disclosed herein. When all 12 foil assemblieshave been spent and dropped, the transport system has provisions fordropping a magazine to the bottom of the accelerator tube, removing theused magazine for reloading and reuse, removing the spent foils fordisposal, and placing a new magazine to continue the cycle.

It should be evident from the illustrations of the operation of themagazine shown in FIGS. 4, 5 and 6 that the choice of suspending shelfis made when the magazine is loaded. If each individual foil rod ismoved one notch toward the coil in FIGS. 4 through 6, the rod at the endfarther from the actuating coil will be dropped first and the foil willdangle from the shelf closer to the coil. The next actuation of the coilwill drop the foil as earlier described and the operation will proceedcyclically through the use of a full magazine of foils dangled from theother shelf. This variation is a matter of choice in loading the foilsand is significant only insofar as use of one shelf or the other maychange the location of dangled foils with respect to the position of thebeam.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A foil assembly for insertion into a beam of charged particles toeffect charge exchange in said beam,- said foil assembly comprising:

. a foil formed of a thin film of an organic polymer, 5 said foil havingan area greater than the crosssectional area of said beam;

a first rod connected to said foil at a first edge thereof, said firstrod being longer than said first edge and projecting beyond said foil ateach end of said first edge to form a pair of first supportingprojections; and

a second rod connected to said foil at a second edge thereof opposite tosaid first edge,said second rod being longer than said second edge andprojecting beyond said foil at each end of said second edge to form apair of second supporting projections.

2. The foil assembly of claim 1 wherein said foil is a thermallystabilized sheet of polymerized p-xylene (poly-paraxylylene) having athickness of 5800 A.

3. The foil assembly of claim 1 wherein said first and second supportingrods are nonferrous metal.

4. A magazine for storing and displaying a plurality of the foilassemblies of claim 1 in a particle accelerator comprising:

a support frame;

a pair of first shelves disposed in a. plane and connected to thesupport frame to support said first rod, said pair of first shelvesincluding a first edge;

a pair of second shelves disposed in the plane of the first shelves andconnected to the support frame to support said second rod of saidstripping foil assembly, said pair of second shelves including a secondedge parallel to said first edge;

an indexing frame disposed in sliding engagement with said pairs offirst and second shelves, said indexing frame including a plurality ofnotches forming with said pairs of first and second shelves a pluralityof enclosures for'stripping foil assemblies, said notches grouped into afirst group and a second group, corresponding notches of said first andsecond groups being spaced relative to each other a distance differingfrom the distance between said first and second shelf edges by the widthof a notch;

ratchet mechanism connected to said support frame and said indexingframe for moving said indexing frame by one-half the width of a notchwith respect to said support frame; and

remotely operated actuating means connected to said ratchet mechanismfor operating said ratchet mechanism from a location remote from saidmagazine.

5. The magazine of claim 4 wherein said support frame and said indexingframe are made of epoxy bonded glass fibers.

, 6. The magazine of claim 4 wherein said remote actuating meanscomprise:

a coil of wire having two ends and disposed to produce a verticallydirected magnetic field on the passage of an electric currenttherethrough; and

a pair of electrical contacts connected one to each of said two ends andadaptable to connect said coil to an external electrical source.

7. An apparatus for inserting a stripping foil into the particle beam ofan accelerator having a vertically directed component of acceleratingmagnetic field; the apparatus comprising:

LII

a thin foil of an organic polymer sized to cover a cross-section of theparticle beam;

a first rod connected to the foil at a side thereof, the ends of saidfirst rod projecting beyond the foil to form a first pair of supportingprojections;

a second rod connected to the foil at a side opposite to the side of thefirst rod, the ends of said second rod projecting beyond the foil toform a second pair of supporting projections;

a support frame including a first pair of shelves separated from eachother by a distance less than the length of said first rod and adaptedto support said first rod by said projecting ends of said first rod,said support frame further including a second pair of shelves separatedfrom each other by a distance less than the length of said second rodand adapted to support said second rod by said projecting ends of saidsecond rod, said support frame disposed adjacent to and above saidparticle beam;

means for sequentially moving said first and second rod with respect tosaid support frame first to move said first rod off said first pair ofshelves to dangle said thin foil in said particle beam, and secondly tomove said second rod off said second pair of shelves to drop said thinfoil out of said particle beam.

8. The apparatus of claim 7 wherein the means for sequentially movingsaid first and second support rod comprise:

an indexing frame including a first notch sized to receive one of saidprojecting ends of said first rod and disposed to open on one shelf ofsaid first pair of shelves to form therewith an enclosure for said oneof said projecting ends of said first rod, said indexing frame furtherincluding a second notch alinged on a first axis with said first notch,sized to receive the other of said projecting ends of said first rod anddisposed to open on the other shelf of said first pair of shelves toform therewith an enclosure for said other of said projecting ends ofsaid first rod, said indexing frame further including a third notchsized to receive one of said projecting ends of said second rod anddisposed to open on one shelf of said second pair of shelves to formtherewith an enclosure for said one of said projecting ends of saidsecond rod, said indexing frame further including a fourth notch alignedon a second axis with said third notch and sized to receive the other ofsaid projecting ends of said second rod and disposed to open on theother shelf of said second pair of shelves to form therewith anenclosure for said other of said projecting ends of said second rod,said first and second axes being parallel to each other and separated bya distance that differs from the separation of said first and secondpairs of shelves by the width of said first notch; and

driving means for producing relative motion of one notch width betweensaid indexing frame and said support frame, whereby operation of saiddriving means when a foil is supported on said first and second pair ofshelves drops one rod from one of said pairs of shelves to dangel thefoil in the particle beam to effect stripping thereof.

9. The apparatus of claim 8 wherein said driving means comprise:

a coil of wire connected to said support frame and said indexing frameand squeezably deformable to produce relative motion therebetween, saidcoil disposed to produce a vertically directed magnetic field on thepassage of an electric current therethrough;

a source of electrical energy connectable to said coil to pass anelectric current therethrough;

means for connecting said source to said coil; a ratchet mechanism forcontrolling the amount and direction of relative motion between saidsupport frame and said indexing frame,

whereby connecting said source to said coil establishes a magnetic fieldtherein to interact with the vertically directed component of theaccelerator magnetic field to produce squeezed deformation of said coiland hence relative motion between said support frame and said indexingframe.

10. The apparatus of claim 8 wherein said support frame and saidindexing frame are epoxy-bonded glass fibers.

11. The apparatus of claim 7 wherein said thin foil is formed ofpolymerized para-xylene (poly-p-xylylene) having a thickness less than6000 Angstroms.

12. The apparatus of claim 7 wherein said support frame is epoxy-bondedglass fibers.

1. A foil assembly for insertion into a beam of charged particles toeffect charge exchange in said beam, said foil assembly comprising: afoil formed of a thin film of an organic polymer, said foil having anarea greater than the cross-sectional area of said beam; a first rodconnected to said foil at a first edge thereof, said first rod beinglonger than said first edge and projecting beyond said foil at each endof said first edge to form a pair of first supporting projections; and asecond rod connected to said foil at a second edge thereof opposite tosaid first edge, said second rod being longer than said second edge andprojecting beyond said foil at each end of said second edge to form apair of second supporting projections.
 2. The foil assembly of claim 1wherein said foil is a thermally stabilized sheet of polymerizedp-xylene (poly-paraxylylene) having a thickness of 5800 A.
 3. The foilassembly of claim 1 wherein said first and second supporting rods arenonferrous metal.
 4. A magazine for storing and displaying a pluralityof the foil assemblies of claim 1 in a particle accelerator comprising:a support frame; a pair of first shelves disposed in a plane andconnected to the support frame to support said first rod, said pair offirst shelves including a first edge; a pair of second shelves disposedin the plane of the first shelves and connected to the support frame tosupport said second rod of said stripping foil assembly, said pair ofsecond shelves including a second edge parallel to said first edge; anindexing frame disposed in sliding engagement with saiD pairs of firstand second shelves, said indexing frame including a plurality of notchesforming with said pairs of first and second shelves a plurality ofenclosures for stripping foil assemblies, said notches grouped into afirst group and a second group, corresponding notches of said first andsecond groups being spaced relative to each other a distance differingfrom the distance between said first and second shelf edges by the widthof a notch; a ratchet mechanism connected to said support frame and saidindexing frame for moving said indexing frame by one-half the width of anotch with respect to said support frame; and remotely operatedactuating means connected to said ratchet mechanism for operating saidratchet mechanism from a location remote from said magazine.
 5. Themagazine of claim 4 wherein said support frame and said indexing frameare made of epoxy bonded glass fibers.
 6. The magazine of claim 4wherein said remote actuating means comprise: a coil of wire having twoends and disposed to produce a vertically directed magnetic field on thepassage of an electric current therethrough; and a pair of electricalcontacts connected one to each of said two ends and adaptable to connectsaid coil to an external electrical source.
 7. An apparatus forinserting a stripping foil into the particle beam of an acceleratorhaving a vertically directed component of accelerating magnetic field,the apparatus comprising: a thin foil of an organic polymer sized tocover a cross-section of the particle beam; a first rod connected to thefoil at a side thereof, the ends of said first rod projecting beyond thefoil to form a first pair of supporting projections; a second rodconnected to the foil at a side opposite to the side of the first rod,the ends of said second rod projecting beyond the foil to form a secondpair of supporting projections; a support frame including a first pairof shelves separated from each other by a distance less than the lengthof said first rod and adapted to support said first rod by saidprojecting ends of said first rod, said support frame further includinga second pair of shelves separated from each other by a distance lessthan the length of said second rod and adapted to support said secondrod by said projecting ends of said second rod, said support framedisposed adjacent to and above said particle beam; means forsequentially moving said first and second rod with respect to saidsupport frame first to move said first rod off said first pair ofshelves to dangle said thin foil in said particle beam, and secondly tomove said second rod off said second pair of shelves to drop said thinfoil out of said particle beam.
 8. The apparatus of claim 7 wherein themeans for sequentially moving said first and second support rodcomprise: an indexing frame including a first notch sized to receive oneof said projecting ends of said first rod and disposed to open on oneshelf of said first pair of shelves to form therewith an enclosure forsaid one of said projecting ends of said first rod, said indexing framefurther including a second notch alinged on a first axis with said firstnotch, sized to receive the other of said projecting ends of said firstrod and disposed to open on the other shelf of said first pair ofshelves to form therewith an enclosure for said other of said projectingends of said first rod, said indexing frame further including a thirdnotch sized to receive one of said projecting ends of said second rodand disposed to open on one shelf of said second pair of shelves to formtherewith an enclosure for said one of said projecting ends of saidsecond rod, said indexing frame further including a fourth notch alignedon a second axis with said third notch and sized to receive the other ofsaid projecting ends of said second rod and disposed to open on theother shelf of said second pair of shelves to form therewith anenclosure for said other of said projecting ends of said seconD rod,said first and second axes being parallel to each other and separated bya distance that differs from the separation of said first and secondpairs of shelves by the width of said first notch; and driving means forproducing relative motion of one notch width between said indexing frameand said support frame, whereby operation of said driving means when afoil is supported on said first and second pair of shelves drops one rodfrom one of said pairs of shelves to dangel the foil in the particlebeam to effect stripping thereof.
 9. The apparatus of claim 8 whereinsaid driving means comprise: a coil of wire connected to said supportframe and said indexing frame and squeezably deformable to producerelative motion therebetween, said coil disposed to produce a verticallydirected magnetic field on the passage of an electric currenttherethrough; a source of electrical energy connectable to said coil topass an electric current therethrough; means for connecting said sourceto said coil; a ratchet mechanism for controlling the amount anddirection of relative motion between said support frame and saidindexing frame, whereby connecting said source to said coil establishesa magnetic field therein to interact with the vertically directedcomponent of the accelerator magnetic field to produce squeezeddeformation of said coil and hence relative motion between said supportframe and said indexing frame.
 10. The apparatus of claim 8 wherein saidsupport frame and said indexing frame are epoxy-bonded glass fibers. 11.The apparatus of claim 7 wherein said thin foil is formed of polymerizedpara-xylene (poly-p-xylylene) having a thickness less than 6000Angstroms.
 12. The apparatus of claim 7 wherein said support frame isepoxy-bonded glass fibers.